Following [1]}, a rotational system \(\mathcal {R}\)  on a finite, connected graph \(G= (V,E)\)  consists of a choice of a cyclic ordering on each set of edges with a common vertex.
A vertex which belongs to a single edge \(e\)  is associated to the ordering \(e \mapsto e\) : we suppose there are two copies of \(e\) . We denote a graph \(G\)  endowed with a rotational system \(\mathcal {R}\)  by \(G^\mathcal {R}\) .
